There has been considerable prior work from this laboratory as well as others documenting age associated left ventricular hypertrophy. It has been deduced that the myocytes must be responsible for much of this hypertrophy, as the increase in collagen and other components of the interstitium is insufficient to account for all of the increase in mass. However, the cellular anatomic basis of age associated cardiac hypertrophy is unknown. We therefore analyzed digitized photomicrographs of isolated single left ventricular myocytes from 2, 8, and 24 month old male Wistar rats. We find that slack sarcomere length and cell width are unchanged over this age range. Cell length increases by 21%, however. Therefore, the average number of sarcomeres per cell length increases with age. This series addition of sarcomeres may have important functional implications as to the mechanism and consequences of age associated myocardial adaptation.